In science, computing, and engineering, a black box is a device, system or object which can be viewed in terms of its inputs and outputs (or transfer characteristics), without any knowledge of its internal workings. Its implementation is "opaque" (black). Almost anything might be referred to as a black box: a transistor, an algorithm, or the human brain.

To analyse something, as an open system, with a typical "black box approach", only the behavior of the stimulus/response will be accounted for, to infer the (unknown) box. The usual representation of this black box system is a data flow diagram centered in the box.

The opposite of a black box is a system where the inner components or logic are available for inspection, which is most commonly referred to as a white box (sometimes also known as a "clear box" or a "glass box").

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The modern term "black box" seems to have entered the English language around 1945. In electronic circuit theory the process of network synthesis from transfer functions, which led to electronic circuits being regarded as "black boxes" characterized by their response to signals applied to their ports, can be traced to Wilhelm Cauer who published his ideas in their most developed form in 1941.[1] Although Cauer did not himself use the term, others who followed him certainly did describe the method as black-box analysis.[2]Vitold Belevitch[3] puts the concept of black-boxes even earlier, attributing the explicit use of two-port networks as black boxes to Franz Breisig in 1921 and argues that 2-terminal components were implicitly treated as black-boxes before that.

The open system theory is the foundation of black box theory. Both have focus on input and output flows, representing exchanges with the surroundings.

The black box is an abstraction representing a class of concrete open system which can be viewed solely in terms of its stimuli inputs and output reactions:

The constitution and structure of the box are altogether irrelevant to the approach under consideration, which is purely external or phenomenological. In other words, only the behavior of the system will be accounted for.

The observed hydrograph is a graphic of the response of a watershed (a blackbox) with its runoff (red) to an input of rainfall (blue).

An observer makes observations over time. All observations of inputs and outputs of a black box can be written in a table with the form:

↓Time

States of input and output

...

... ...

...

... ...

in which, at each of a sequence of times, the states of the box’s various parts, input and output, are recorded. Thus, using an example from Ashby, examining a box that has fallen from a flying saucer might lead to this protocol:[4]

↓Time

States of input and output

11:18 a.m.

I did nothing—the Box emitted a steady hum at 240 Hz.

11:19

I pushed over the switch marked K: the note rose to 480 Hz and remained steady.

When the observer (an agent) can also do some stimulus (input), the relation with the black box is not only an observation, but an experiment.

Thus every system, fundamentally, is investigated by the collection of a long protocol, drawn out in time, showing the sequence of input and output states. From this there follows the fundamental deduction that all knowledge obtainable from a Black Box (of given input and output) is such as can be obtained by re-coding the protocol (the observation table); all that, and nothing more.[4]

If the observer also controls input, the investigation turns into an experiment (illustration), and hypotheses about cause and effect can be tested directly.

When the experimenter is also motivated to control the box, there is an active feedback in the box/observer relation, promoting what in control theory is called a feed forward architecture.

Black box theories are things defined only in terms of their function.[9][10] The term black box theory is applied to any field, philosophy and science or otherwise where some inquiry or definition is made into the relations between the appearance of something (exterior/outside), i.e. here specifically the thing's black box state, related to its characteristics and behaviour within (interior/inner).[11][12]

Specifically, the inquiry is focused upon a thing that has no immediately apparent characteristics and therefore has only factors for consideration held within itself hidden from immediate observation. The observer is assumed ignorant in the first instance as the majority of available data is held in an inner situation away from facile investigations. The black box element of the definition is shown as being characterised by a system where observable elements enter a perhaps imaginary box with a set of different outputs emerging which are also observable.[13]

In computing in general, a black box program is one where the user cannot see the inner workings (perhaps because it is a closed source program) or one which has no side effects and the function of which need not be examined, a routine suitable for re-use.

Also in computing, a black box refers to a piece of equipment provided by a vendor, for the purpose of using that vendor's product. It is often the case that the vendor maintains and supports this equipment, and the company receiving the black box typically is hands-off.

In neural networking or heuristic algorithms (computer terms generally used to describe 'learning' computers or 'AI simulations'), a black box is used to describe the constantly changing section of the program environment which cannot easily be tested by the programmers. This is also called a white box in the context that the program code can be seen, but the code is so complex that it is functionally equivalent to a black box.

In physics, a black box is a system whose internal structure is unknown, or need not be considered for a particular purpose.

In mathematical modeling, when the primary goal is the most accurate replication of data, regardless of the mathematical model structure, a black-box modeling approach is useful. If the inputs and outputs are known the model can be inferred. Black-box modeling is a trial-and-error method, where parameters of various models are estimated, and the output from those models is compared to the results with the opportunity for further refinement.[19]

In cryptography to capture the notion of knowledge obtained by an algorithm through the execution of a cryptographic protocol such as a zero-knowledge proof protocol. If the output of an algorithm when interacting with the protocol matches that of a simulator given some inputs, it 'need not know' anything more than those inputs.

Black Box theory is, however, even wider in application than these professional studies:

The child who tries to open a door has to manipulate the handle (the input) so as to produce the desired movement at the latch (the output); and he has to learn how to control the one by the other without being able to see the internal mechanism that links them. In our daily lives we are confronted at every turn with systems whose internal mechanisms are not fully open to inspection, and which must be treated by the methods appropriate to the Black Box.

(...) This simple rule proved very effective and is an illustration of how the Black Box principle in cybernetics can be used to control situations that, if gone into deeply, may seem very complex.
A further example of the Black Box principle is the treatment of mental patients. The human brain is certainly a Black Box, and while a great deal of neurological research is going on to understand the mechanism of the brain, progress in treatment is also being made by observing patients' responses to stimuli.

In aviation, the flight recorder is often called a "black box", especially by the media, although it is usually bright orange to facilitate its discovery after a crash. In an airplane or helicopter, the flight recorder consists of two independent devices that are usually combined in a single unit. The flight data recorder logs information about controls and sensors, and the cockpit voice recorder preserves the most recent sounds in the cockpit, including conversation of the pilots. If an accident happens, investigators can use the recordings to assist in the investigation.

In amateur radio the term "black box operator" is a disparaging or self-deprecating description of someone who operates factory-made radios without having a good understanding of how they work. Such operators do not build their own equipment (an activity called "homebrewing") or repair their own "black boxes".[22]